The NAVSTAR Global Positioning System (GPS) comprises a constellation of satellites and a control segment whose function it is to monitor and update the clocks, orbits, and navigation messages of these satellites. GPS receives receive radio transmissions from satellite-based radio navigation systems and use those received transmissions to determine the location of the receiver. The location of the receiver may be determined by applying the well-known concept of trilateration of the distances from the receiver to satellites having known satellite locations.
Generally, each satellite in a satellite-based radio navigation system broadcasts a radio transmission, which contains its orbit parameters and a ranging signal. Together, these two quantities allow the location of the satellite to be known as a function of time. Each of the orbiting satellites in the GPS system contains four highly accurate atomic clocks: two Cesium and two Rubidium. These clocks generate precisely timed binary codes (also known as a pseudo random noise “PRN,” or pseudo noise “PN” code) that are transmitted to earth. The PN codes identify the specific satellite in the constellation. Each satellite transmits a set of digitally coded ephemeris data that completely defines the orbit of the satellite. This ephemeris data provides an accurate indication of the position of the satellite as a function of time.
Atomic clocks are very precise; however, a slight error (generally known as clock drift) may occur in the clocks over time resulting in satellite clock errors of about 15 ns per day with corresponding range errors of about 5 meters. In order to compensate for the error, the accuracy of the satellite atomic clocks are continuously monitored from ground stations in the GPS control system and any detected errors and drift in the clock of the satellites may be calculated and transmitted by the satellites as part of a navigation message in the form of three coefficients of a second-degree polynomial.
In the case of GPS, there is nominally a constellation of 24 operational satellites above the Earth. Each satellite has individual PN codes, a nearly circular orbit with an inclination of 55 degrees to the equator with a height of approximately 10,900 nautical miles (20,200 kilometers) above Earth and an orbital period of approximately 12 hours. Each GPS satellite transmits a microwave radio signal composed of two carrier frequencies modulated by two digital codes and a navigation messages. The two carrier frequencies are referred to as the “L1” and “L2” carriers and are transmitted at 1,572.42 megahertz (MHz) and 1,277.60 MHz, respectively. The two GPS codes are called the coarse acquisition (C/A-code) and precision (P-code). Each code consists of a stream of binary digits, zeros and ones, known as “chips.” Both the C/A-code and P-code are generally referred to as a PN code because they look like random noise-like signals. Presently, the C/A-code is modulated only on the L1 carrier while the P-code is modulated on both L1 and L2 carriers.
The C/A-code is a length 1023 sequence and has a chip rate of 1.023 MHz. The code repeats itself nominally every millisecond. Each satellite is assigned a unique C/A-code, which enables a GPS receiver to identify which satellite is transmitting a particular code. The C/A-code range measurement is relatively less precise when compared to the P-code but it is also less complex and available to all users. The P-code is mostly limited in use to the United States government and military. At present an encrypted version of the P-code is transmitted. This encrypted version of the P code is known as the Y code.
Each satellite also transmits a GPS navigation message that is a data stream that further modulates the L1 carrier as binary bi-phase modulation at 50 bits per second (bps). The navigation message contains, along with other information, the ephemeris data which defines the coordinates of the GPS satellites as a function of time, the satellite health status, the satellite clock corrections, and the satellite almanac. Each satellite transmits its own navigation message with information on the other satellites, such as the approximate location and health status.
To receive the signals transmitted by the satellites, a GPS receiver generates and aligns replicas of the code and carrier signals contained in the received signal for each received satellite. The relative distances to a plurality of satellites are measured by observing what phases of the code replicas correlate or align with the incoming GPS signal. These relative distances are used to solve for position. As the clock in a standalone GPS receiver has an arbitrary timing relationship, with respect to the common GPS time of the satellites, it is necessary to solve for the receiver time in addition to the three spatial coordinates of user. Hence, four satellite range measurements are typically required to compute position. Beyond the random timing offset of the clock, the clock frequency of the MS is inevitably offset with respect to the GPS frequency in standalone applications.
Besides accuracy, another problem associated with the error of the MS clock reference relative to the GPS satellite clocks is the resulting acquisition time for the GPS receiver commonly known as the time to first fix (TTFF). For many applications, such as Emergency 911 (E911) positioning, a GPS receiver must be able to provide a position solution in a short period of time after being commanded to compute this position. Unfortunately, the MS clock reference can have substantial frequency uncertainty. The large frequency uncertainty can cause significant degradation on TTFF performance. Additionally, uncertainty in time can require the examination of many code phases. GPS satellites positioned above the earth's atmosphere, it is not always possible for a GPS receiver to receive accurate transmissions from the required number of GPS satellites necessary to calculate the position of the GPS receiver.
A GPS receiver can at time be located so that the GPS receiver can not receive strong GPS signals from at least four satellites. For example, when the GPS receiver is located indoors, the GPS signals by the GPS receiver can be greatly attenuated. The receiver may not be able to receive the at least four GPS signals, plus it can become very difficult to estimate the time of arrival of the attenuated GPS signals.
Since the inception of GPS, methods have been, and are still being, developed to reduce errors and to enhance the accuracy of the GPS systems. Further, many different methods are being implemented to provide alternative means for providing the GPS receiver with information concerning unknown variables or inaccuracies in the system such that it is not always required for the system to receive satellite transmission signals from all the satellites or to receive accurate transmission data.
It is desirable for to have a system and method for aiding mobile subscriber (MS) position estimation.